#include <cstdio>
#include <algorithm>
#include <cstring>
using namespace std;
const int N=105;
double x[N];
double y[N];
int main(void){
    int n;
    while(~scanf("%d",&n)){
        memset(x,0,sizeof(x));
        memset(y,0,sizeof(y));
        for(int i=0;i<n;i++){
            scanf("%lf%lf",&x[i],&y[i]);
        }
        double res=0;
        for(int i=0;i<n-1;i++){
            res+=(x[i]*y[i+1]-x[i+1]*y[i]);
        }
        printf("%.1lf\n",res/2.0);
    }
    return 0;
}
